Water vapor is a self regulating, repeating process. If you increase the water vapor by adding heat, you increase the number of rain storms that remove that heat. Overall, the average remains relatively constant with only the frequency increasing. So, yes it does increase the planet's insulation but only while it's in the air.
CO2, methane and the others to much lesser degrees, stay in the atmosphere constantly. They build up and cause a rise in the baseline from which water vapor's changes oscillate around.
I commend you on your awareness of these events but I would caution you to keep an eye on the big picture.
An analogy I often use is that of a steady hose filling a swimming pool. If there are some pinholes at various heights in the side of the pool, the water will eventually equalize wherever the input/outflow balances. Closing one hole will raise that level to a new stable level. Allowing leaves to fall into the pool will constantly block an increasing number of holes and make the level constantly rise. It's not a perfect analogy but it works for the most part.
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